This invention relates to a multilayer composite material and method for evaporative cooling of a person and to garments and other articles made from the multilayer composite material.
The use of phase change materials or PCM""s for thermal storage has long been known. In many materials much more heat can be stored as latent heat of phase change than as sensible heat. One of the earliest known applications of this principle is the use of ice as a thermal storage medium for perishable foods. Some of the phase transitions that involve significant amounts of latent heat are solid-to-liquid or liquid-togas phase changes or, in certain materials, solid-to-solid phase changes. Another subgroup of PCM""s uses reversible exothermic reactions, such as hydration-dehydration or solution-precipitation reactions, for heat storage. The latent heat of phase change can be used for heating or cooling depending on whether the phase change is exothermic or endothermic. In most materials, the phase changes are reversible so that the latent heat storage can be used for either heating or cooling depending on the temperature conditions.
PCM""s have recently been applied to personal heating and cooling devices. U.S. Pat. No. 4,856,294 to Scaringe et al. describes a Micro-Climate Control Vest which contains a PCM with a solid-to-liquid phase change between 60 and 90xc2x0 F. as a cooling medium. The vest may also have an optional second PCM layer of ice and an optional outer insulation layer. Because of the inherent rigidity of the PCM in its solid state, the inner liner which contains the PCM is divided into individual compartments separated by vertical seams that provide folding points in the vest. This provides very limited flexibility in the vest, making it impractical for use during intense physical activity where the rigidity would be a hindrance.
U.S. Pat. No. 4,894,931 to Senee et al. describes a Heating Device for warming body parts, such as the soles of the feet. The device uses a battery powered electric heater and a PCM salt. The PCM salt serves as a heat storage medium and as a temperature regulator for the resistance heater since it can absorb a lot of heat without rising above its melt temperature. This device is described as having a rigid base, which along with the inherent rigidity of the solid PCM salt, would make it unusable for applications where flexibility is important.
U.S. Pat. Nos. 4,572,158 to Fiedler and 4,077,390 to Stanley et al. both describe heating pads for warming body parts that use a supercooled PCM salt solution for heat storage. The PCM can be dissolved by placing the pad in boiling water. Once the PCM is liquefied, it can be cooled to room temperature without solidifying. A trigger is used to activate the PCM, causing an exothermic crystallization which releases heat at about 135xc2x0 F. The device is sold with a cloth or neoprene cover to prevent burns when it is placed against the skin. These heating pads are made in small sizes for spot heating parts of the body. Because the solid PCM salt is very rigid, the pad cannot be used for heating large areas of the body where the rigidity would be a hindrance to movement.
U.S. Pat. No. 4,756,958 to Bryant et al. describes a fiber with enhanced thermal storage properties and fabrics made therefrom. The fiber described contains microcapsules of PCM ranging from 1-10 microns in size. While these fibers provide the sought after flexibility, they do not contain enough PCM to have sufficient thermal mass to be of practical use in heating or cooling.
U.S. Pat. No. 4,851,291 to Vigo et al. describes another method of making fibers with thermal storage properties by filling the core of a hollow fiber with a PCM or absorbing a PCM onto the surface of a non-hollow fiber. The PCM""s described include cross-linked polyethylene glycol and plastic crystals that have a solid-to-solid crystalline phase change. These fibers can also can be made very flexible, but their geometry does not allow absorption of enough PCM into the containment material to be of practical use in heating or cooling.
From the preceding discussion it can be seen that the prior art, in all of its attempts, has failed to provide a flexible composite material with thermal storage properties that at the same time provides: 1) sufficient thermal mass for heating or cooling large areas of the body, and 2) enough flexibility to be fashioned into garments or other articles suitable for wear during vigorous physical activity where rigidity would be a hindrance to movement. With this in mind, the objective of the present invention is to provide a highly flexible composite material that contains a high thermal mass of a phase change thermal storage material. The phase change thermal storage material is dispersed within the composite material such that the entire composite is flexible when the phase change material is in a high energy or low energy state. In various embodiments, the composite material can be made to heat or cool the body or to act as a thermal buffer to protect the wearer from changing environmental conditions. To help with this objective, the material may also include an external thermal insulation layer and/or an internal thermal control layer to regulate the rate of heat exchange between the composite and the skin of the wearer. Another important objective is to provide a flexible thermal storage composite material that can easily be fashioned into garments or other articles, such as blankets, that can be worn for their thermal properties.
Other secondary objectives of the invention are to provide a flexible thermal storage composite material that also provides 1) a path for evaporation or direct absorption of perspiration from the skin of the wearer for improved comfort and thermal control, 2) heat conductive pathways within the material for thermal equalization, 3) surface treatments for improved absorption or rejection of heat by the material, and 4) means for quickly regenerating the thermal storage capacity for reuse of the material.
In keeping with these objectives, the simplest embodiment of the present invention provides a composite material that has a phase change material incorporated into a flexible matrix material. In a second embodiment, the composite also includes an external thermal insulation layer and an internal thermal control layer to modify the surface temperature experienced by the skin of the wearer. Other embodiments feature moisture wicking components or other components that add different thermal properties to the composite. In each embodiment, the PCM may be chosen to provide heating, cooling or environmental buffering.
Exemplary applications of the composite materials are also described which take advantage of the composite""s thermal characteristics. The examples described include a diver""s wet suit, ski boot liners, thermal socks, gloves and a face mask for cold weather activities, and a metabolic heating or cooling blanket useful for treating hypothermia or fever patients in a medical setting.
In one embodiment, a multilayer composite material and method for evaporative cooling of a person employs an evaporative cooling liquid that changes phase from a liquid to a gaseous state to absorb thermal energy. The evaporative cooling liquid is absorbed into a superabsorbent material enclosed within the multilayer composite material. The multilayer composite material has a high percentage of the evaporative cooling liquid in the matrix. The cooling effect can be sustained for an extended period of time because of the high percentage of phase change liquid that can be absorbed into the superabsorbent. Such a composite can be used for cooling febrile patients by evaporative cooling as the evaporative cooling liquid in the matrix changes from a liquid to a gaseous state thereby absorbing thermal energy. The composite can be made with a perforated barrier material around the outside to regulate the evaporation rate of the phase change liquid. Alternatively, the composite can be made with an imperveous barrier material or semipermeable membrane on one side to prevent the liquid from contacting the person""s skin. The evaporative cooling liquid in the matrix can be recharged by soaking the material in the liquid. The multilayer composite material can be fashioned into blankets, garments and other articles.